Two-phase flows, such as gas-solid two-phase flows, are widely applied in production practices of industries, such as electrical, metallurgical and pharmaceutical industries. For example, in the fields of electrical and metallurgical industries, pneumatic conveying of pulverized coal is a typical gas-solid two-phase flow, which can increase combustion efficiency, improve production environment and reduce labor intensity. Thus, the pneumatic conveying of pulverized coal facilitates a solution to the problems, such as an increase in waste of coal combustion as an energy source and deterioration of the environment.
In the electrical industry, an appropriate ratio of coal distribution to combustion-supporting air is one of the essential conditions for optimizing combustion of large scale fired boilers. For a coal-fired boiler, inconsistent coal dust flows in a pipeline among a combustor will influence the air-to-coal ratio of the combustor, resulting in a lowered combustion rate, increased level of unburned carbon and high NOx emission, as well as clogged primary air ducts, accidental fires, and even local clogging of the coke in severe cases.
In the metallurgical industry, pulverized coal injection is a significant technical method of reducing costs and improving product quality in iron making. In the case of an uneven coal distribution among different tuyeres, the combustion cannot be well organized. In the case of a clogged injection pipe, the clogging may lead to an overflow of pulverized coal from the tuyere, tuyere damage or even an explosion of pulverized coal in warm-air pipes, which impacts safe production and the regular functioning of furnaces.
Therefore, the flow parameters of the pulverized coal-air two-phase flow, which flows in a pneumatic conveying pipeline, must be measured to monitor the flow state of the gas-solid two-phase flow in the pneumatic conveying pipeline or to provide alerts of accidents from conveying pulverized coal.
However, due to the non-homogeneous concentration distribution and the complex flow pattern of the gas-solid two-phase flow, parameter measurements for the solid phase is a problem known in the art, which the electrical and metallurgical industries have attempted to address, but have not yet been successful.
At present, there are two main classes of methods for measuring pulverized coal-air flow state, one of which is a contact method. With this method, sensors are directly placed in the pipelines for measurement of a flow field. However, the contact method tends to disturb the flow field, which makes real flow state unavailable and can be a cause of fault. A disadvantage of the contact method is that the sensors are liable to be damaged due to direct erosion by the fluids. The second method is a non-contact method, which includes a temperature difference method and an optical detection method. The principle of the temperature difference method is based on the fact that the temperature of the pulverized coal from the pulverized coal injection system is higher than both the environmental temperature and the temperature of compressed air after dry and cold dehydration. A state diagnosis is then performed according to a variation in the temperature difference before and after pulverized coal is clogged in the pipeline. For this method, summarizing rules for determining temperature difference between pulverized coal and conveying pipeline before and after clogging, and diagnosing a flow state of pulverized coal in an outdoor environment is difficult. Even in an enclosed environment, determining the temperature difference determination rules is difficult. With the optical detection method, an optical detector is equipped in a cone space in front of the watch hole of the tuyere and is used to monitor the state of a blast-furnace tuyere. For this method, in case of a furnace blowing down or tuyere maintenance, the device needs to be removed and mounted, which is inconvenient. Further, the optical detection method requires that the detected area has good light transmittance, which is a challenge for the optical detection method when applied in a high concentration dense coal powder conveying system.
Therefore, measuring flow parameters of a two-phase flow in a convenient and accurate way is a technical difficulty, which needs to be solved urgently.
In production practice, flow parameters of a two-phase flow that need to be measured include a volume concentration of a solid phase, a velocity of the two-phase flow, a mass flow rate and a temperature. To date, the volume concentration of the solid phase, the velocity of the two-phase flow and mass flow rate of a working medium phase are flow parameters that cannot be accurately and reliably measured.